Bring back that router/modem and ask for a Standard non wireless Modem and get your own router so theres no need to dick with bridged mode/pppoe
Get a netgear N900, probably one of the better routers you can get right now, stay the fuck away from Cisco/Linksys shit its all garbage as of the past few years…
The UBEE router probably does not have the option/protocol in it that helps with having multiple xboxes on the network. Due to the way Microsoft handels authentication etc I guess running multiple xboxes on a network in general can cause havoc between them… some routers (n900 is one of them) has whatever stupid option it is that helps with this